The present invention relates to an automatic transmission control device for controlling an automatic transmission that includes a pressure regulating device that regulates an engagement pressure of at least one friction engagement element, and is capable of transmitting power to an output shaft, which is input from a power source to an input shaft through a change of a shift speed by controlling the pressure regulating device to switch an engagement state of the friction engagement element, wherein when the shift speed is changed while torque is applied to the input shaft, the automatic transmission changes the shift speed through two phases that respectively are a torque phase in which transmission of the torque is changed to be made in accordance with a shift speed after shifting, and an inertia phase in which a rotation speed of the input shaft is changed to a rotation speed corresponding to the shift speed after shifting. The present invention also relates to a transmission device and a power output device.